The interaction of the urokinase-type plasminogen activator (uPA) with cell surfaces is exclusively mediated by its glycolipid-anchored receptor (uPAR) to which uPA binds with high affinity.
uPAR is localised to the outer layer of the cell membranes via glycosyl phosphatidylinositol linkage (GPI anchor). It is a cysteine-rich heavily glycosylated protein composed of three homologous domains. uPA is composed of the catalytic C-terminal serine protease domain and a modular N-terminal part including a growth factor-like domain (GFD; aa 1-49) and a kringle domain (aa 50-135). The interaction between uPA and uPAR is primarily mediated by residues 19-31 of the GFD of uPA.
Since proteolytic degradation of the extracellular matrix has an established role in tumour invasion and metastasis, uPAR represents a potential target for diagnostic contrast agents. uPAR appears to be up-regulated in vivo in most human carcinomas examined to date, specifically, in the tumour cells themselves, in tumour-associated endothelial cells undergoing angiogenesis and in macrophages. uPAR overexpression in cancer patients is present in advanced disease and has been correlated with poor prognosis in numerous human carcinomas. The fact that uPAR expression is up-regulated only in pathological states involving extracellular matrix remodelling and cell motility such as cancer makes it an attractive marker for diagnosis.
WO 01/25410 describes diagnostically or therapeutically labelled uPAR-targeting proteins and peptides. The peptide or protein comprises at least 38 amino acid residues, including residues 13-30 of the uPAR binding site of uPA.
U.S. Pat. No. 6,277,818 describes uPAR-targeting cyclic peptide compounds that may be conjugated with a diagnostic label. The peptides are based on the amino acid residues 20-30 of uPA.
U.S. Pat. No. 6,514,710 is also directed to cyclic peptides having affinity for uPAR. The peptides may carry a detectable label. The peptide comprises 11 amino acids joined by a linking unit.
Ploug et al. in Biochemistry 2001, 40, 12457-12168 describes uPAR targeting peptides but not in the context of imaging, including amino acid sequences as described in the present document.
The efficient targeting and imaging of uPAR demands a selective high-affinity vector that is chemically robust and stable. These stringent conditions are met by the contrast agents of the invention.